1. Field of the Invention
This invention relates to a traction control system for a vehicle which controls the slip of the driving wheels of a vehicle.
2. Description of the Prior Art
There has been known a traction control system for a vehicle which controls the slip of the driving wheels of the vehicle by reducing the output of the engine of the vehicle and/or applying brakes for the driving wheels. In such a traction control system, a target slip value (a target slip ratio or a target amount of slip) is determined according to the friction coefficient of the road surface and when the slip value of the driving wheel relative to the road surface exceeds the target slip value, the driving torque acting on the driving wheel is reduced by reducing the engine output torque and/or applying the brake to the wheel so that the slip value of the driving wheel relative to the road surface converges on the target slip value.
When the driving torque is reduced by reduction of the engine output torque, the reduction of the engine output torque is effected, for instance, by controlling the amount of intake air introduced into the combustion chambers of the engine through the intake passage thereof. For example, in the traction control system disclosed in Japanese Unexamined Patent Publication No. 63(1988)-109248, a sub-throttle valve is provided in the intake passage in addition to a throttle valve which is interlocked with an accelerator (will be referred to as "the main throttle valve", hereinbelow), and the opening degree of the sub-throttle valve is controlled to converge the slip value of the driving wheel on the target slip value.
In the traction control system where the driving torque acting on the driving wheels is reduced by control of the opening degree of the sub-throttle valve, the sub-throttle valve is normally held at a fully open position, and the sub-throttle valve is closed to an opening degree according to the slip value of the driving wheels relative to the road surface when the slip value of the driving wheels exceeds the target slip value. However if the sub-throttle valve is directly closed from the fully open position to the opening degree determined according to the slip value, it takes a long time and the response of the sub-throttle valve at the beginning of the traction control delays. This problem may be overcome by first moving, upon starting of the traction control, the sub-throttle valve to the opening degree corresponding to the opening degree of the main throttle valve at that time and then to the opening degree determined according to the slip value.
However, this approach gives rise to another problem that, if the opening degree of the main throttle valve upon starting of the traction control is relatively small (e.g., not larger than 50%), the amount of intake air is abruptly reduced in response to moving the sub-throttle valve to the opening degree determined according to the slip value and the vehicle cannot be properly accelerated.